Frequency modulator



l Fig.

J. L. HATHAWAY FREQUENCY MODULATOR Filed Nov. 22, 1940 3 Shegt's-Sheet 1 ATTO R N EY J. L. HAMA Ar BY W/KM/f/ s sheets-sheet 2 INVENTOR WAY ATTORNEY Feb- 8, 1944. J. HATHAWAY FREQUENCY MODULATOR F'iled Nov. 22, 1940 AAA ' Feu s, 1944.* J.4 L HATHAwAY 2,341,040 v vFRF'QULNGY MoDULA'roR l Filed Nov. 22, 1940 s sheds-sheet s .l ,1| f gialli @zdf 132 TT /06 va 1' /02 FII-Ill.'

foo f n v0 .2 E: @Fg/#gm A ilFl l ATTORNEY Patented Feb. 8, 1944 FREQUENCY MODULATOR Jarrett Lewis Hathaway, Manhasset, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application November 22, 1940, Serial No. 366,546

'14 Claims. (Cl. 179-1715) v This application concerns a. new and improved method of and means for modulating thei frequency of oscillatory energy in accordance with control potentials. In the method and means of the application an oscillator has its frequency of operation controlled in accordance with changes in the modulating voltage. The oscillator may be of the electron-coupled type or other types of oscillators may be used.

In my system, oscillation generation, modulation, and, if desired, frequency multiplication, is accomplished in a single tube.

The principle involved here is the same as that' involved in Townsends United States'application #346,309, led July 19, 1940, now U. S. Patent No. 2,288,375, dated June 30, 1942. In the said Townsend application. the controlled reactance comprised a separate tube coupled to the genwhich acts to degenerate the input impedance also acts regeneratively to produce self oscillations. Thus, an oscillating tube may be controlled in frequency by shifting the potential of any or all of its elements to produce a shift of transconductance.

4 Although. as will be seen, my system is simple in nature and circuit arrangement, it has proved very stable in operation. Although I make no use of special apparatus such as automatic irequency control means, compensating reactances, etc., my system has been found to operate over a period of six or eight hours with a, total maximum deviation of considerably less than 1500 cyclesA across the grid-to-cathode impedance and the voltage thereacross controlled by a resistance inf the cathode return degeneratively connected.

- condenser CI, blocking and coupling condenser h' In Fig. 2,*a pentode tube is used and the variable reactance is inductive with modulation on the control grid whereas in Fig. l modulation is on the anode. y

Figs. 3 and 4 include with the generator and modulator the audio-frequency source and ampliiler. In Fig. 3, a pentode is used with suppressor grid modulation and the output is electronically coupled to the generator. Fig. 4 is similar to Fig. 3 but modulation is applied to the screen grid. Fig. 3a is a modlcation of the arrangement of Fig. 3. In Fig. 3a, a modulation potential amplier is omitted.

In Fig. l, an electron discharge tube I0 has its control grid l and its anode I6 with its cathode 8 connected in an oscillation generating circuit comprising a grid leak resistance RI, grid C3, inductance LI tuning condenser CI, and controlled condenser C2. When the tube electrodes are energized, self-oscillations are sustained by virtue of the inductive coupling in the two sections of LI on opposite sides ofthe cathode tap thereto. This operation being well known inthe art needs no further description here.

The condenser C2 is connected between the grid 4 and cathode 8 of tube l0 and this connection is extended to ground and minus B through the resistance R2. A positive direct-current potential is supplied to the anode I 6 by way of inductor L1 and a. part of inductance LI. Modulating potentials are supplied by way of this same circuit between the anode and cathode of'tahe tube i Il across condenser C3.

The object of the circuit illustrated in Fig. 1 is to provide a method and means by which the frequency of the oscillations developed in the tube KIll and the circuit comprising elements Ll and C2 can be varied in accordance with changes when operating at a frequency of 39.64 megacycles. the F. C. C.

In describing my invention, reference Awill bel made to the attached drawings wherein:

This is`well within the requirements of Y .i

capacity of condenser C2 are Figs. l to 4, inclusive, each shows a frequency modulation system arranged in accordance with my invention. In Fig. 1', a. triode is arranged for the production of oscillations anda capacity,

thej-valueof which iscontrolled. is connected n in a voltage supplied at the terminals marked "Modulation inputf? This frequency variation is. in part at least, the result of changes in the effective capacity of the condenser C2. which is included in the oscillation generating circuits and the changes lnthe caused in the following manner.

The charge which condenser C2 assumes is'a function of, among other things, the voltage appearing acrossit. The voltage appearing across condenser. C2 is a function of the transconductance of tube l0.' Through the' action of' tube I when connected as shown, a voltage is generated across resistance R2 which is a fraction of, and in phase with, the voltage appearing across C2 and R2 in series due to the oscillations generated in I0.. The effective voltage applied to condenser C2 is this oscillator voltage minus that portion of the voltage (degenerative) appearing across R2 which is due to current flowing in the output of tube I0.

More in detail, radio-frequency current produced in the oscillator flows through the circuit including C2 and R2 in series. This current flows all of the time (when the oscillator is operating) irrespective of the operating potential on tube I0. Also, flowing through R2 is the radio frequency flowing through the plate-tocathode path of tube l0 which `includes R2. When tube I0 conducts, a voltage is produced across R2 which is due to the plate-to-cathode current in tube I0, which current is in phase with the potential diierence between grid and cathode. This grid to cathode voltage will lag the generated voltage atA the grid of tube I0 by an amount determined by the relative impedances of C2 and R2. If the impedance of R2 is kept small in proportion to that of C2, a relatively large part of the voltage across R2 due tothe action of tube` I IJ will be in phase vwith the voltage at the grid of tube I0. Since this voltage across R2 is applied to the terminal of C2 opposite to that at which the voltage at the grid of tube Ill is applied, the effective drop across C2 is reduced by the action of tube l0. As a consequence, the value of C2 depends on this voltage`difference and the latter is modulated to frequency modulate the oscillations.

The voltage drop in R2 is variable and is varied as a function of the modulating voltage by varying the conductance of I0. Since the magnitude of this voltage across R2 is a function of ther mutual conductance of I0, the effective capacity of C2 and hence the frequency of the combination of Ll and C2 are also functions of the mutual conductance of tube I0. In the diagram, control of the mutual conductance of tube Il) is obtained by voltages applied to the anode direct-current circuit.

. When' the tube Ills mutual conductance is raised by the modulating potentials, the effective capacity of C2 is lower (because the radiofrequency voltage across R2 increases and that across C2 goes down) and the effective capacity in the oscillator circuit is lower. This causes the oscillator frequency to increase when the transconductance of tube I0 is increased.

',Obviously, since the frequency of operation of tube I0 is determined in part by its reactance circuits, the frequency of operation thereof can be controlled and if the control potential is characteristic of voice or television signals, the frequency modulation of the oscillations will correspond to said signals.

A feature of my method andsystem is that of C2 increases the obtainable deviation and the condenser C2 can be rightly called the swing control condenser. .The lower capacity range isv limitedin my system only by the extent to which the conductivity of the tub'e can be raised, asH

suming cathode and grid potential to be in phase. In practice, deviation of the oscillations over a wide range with a comparatively-stable mean frequency can be attained. v I

The arrangement in Fig. 2 uses the principle involved in Fig. l but differs from Fig. 1 in the following important respects. In Fig. 2, a pentode tube-.I0 is used in place of the triode of Fig. l. The swing condenser C2 is replaced by an inductance L which serves the same purpose in the circuits as C2. The variable reactive effeet in this modification is inductive so that, as will be seen later, the frequency swing is vdown when the transconductance 0f tube I0 is increased, this being opposite to the operation of' the arrangement of Fig l. C6 acts merely as a blocking condenser. Modulation is applied to the control grid 4.

The operation of the arrangement of Fig. 2, in general, is similar to operation of the modification shown in Fig. l. However, in Fig. 2, the inductance L and the resistor R2 are in the oscillator radio-frequency current path and the voltage across L is leading the current Whereas in Fig. 1 the voltage across C is lagging the current. Now, the ability of' inductance L to take this lagging current is a function in part at least of the voltage across L. Consequently, when the tube 10's conductance is low, the drop across R2 is small, the voltage across L is high, and its effective inductance is low and as the tube becomes more conductive, the voltage across L becomes lower and its effective inductance in creases. The frequency of the oscillations generated in this modification consequently decreases With increase in tube transconductance.

The frequency modulated output may be taken from the system of Figs. l and 2 by coupling the tank circuit inductance LI to an output circuit I9. Any approved coupling and output circuit maybe used here andthe output may be amplied, multiplied, limited, etc., before transmission.

' wide band modulation is desired, it is preferable to keep these condensers small and increase the size of swing condenser C2. However, it is usually necessary to keep C2 much smaller than the total of C4 and C2| in order to secure stable oscillation. C2 is the swing condenser and in this respect the system is similar to that shown in Fig. l. Modulation is accomplished by varying the potential on the suppressor grid 24 by means described more in detail later to vary the tube transconductance. The radio voltage at the grid 4 and cathode 8 are sufficiently in phase to provide the desired amount of degeneration and the voltage across C2 is varied as the drop in R2 varies with variations in the transconductance of tube I0. Simultaneously, oscillation occurs due to regeneration between' grid and plate circuits in the conventional manner.

If less frequency shift is desired, R2 may bey omitted. The voltagediiference across the portion of the inductance LI in the cathode return 2,341,640 circuit'vallows sor-ney degree .ofcontrol to be hadthroughv cathode degeneration. VIn .Fig`. 3d ,is

shown this latter arrangement. l

Returning toFig. 3, modulating potentials are supplied from: any source such as a mike or a television scanner to the primary winding of. a transformer'30, the .secondary winding of which is connected inpush-pull relation. to the grids 32 and 34 'of an amplifier tube 36. The anodes 38 and 40l of tube l36 are coupled in push-pull relation to the primary winding of a transformer 42,the secondary winding of which -is connected to the input electrodes of a triofde portion of a triode-diode type tube 46. The output j oflthe amplifier system is supplied from the anode 48 of tube 46` by way of a coupling condenser50 to the suppressor grid 24 of tube III to controlits transconductance for the purpose specified` in detail above.v -This circuit is bypassed at a point close to tube I by aradio-frequency bypass condenser 52. f'

A modulating potential'wave modifying means is included between the secondary winding of thel` transformer 42 and the grid 44 of tube 46. This means comprises an attenuator resistor 56, which allows the low and medium frequency to pass to a limited degree shunted bycond'enser 54,which allows high frequencies to pass unattenuated by virtue of its reactance` characteristic. Resistances 5I!` and -60 are connected from the opposite ends of condenser 54 to the low potential end of the secondary winding for impedance matching purposes. In this circuit the high frequency modulating potentialsare exalted; that is, are relatively increased in amplitude with respect to the lower frequency modulating potentials. Another manner of stating this is to say that the low frequency modulating potentials areattenuated to an extent greater than that to which the higher modulating potentials are attenuated.

Thedirect-current supply circuits for the tubes 36 and 46 areA shownand will be understood by those skilled in the art, it is believed. However, it will be noted that the bias for the control electrodes 32 and 34 of tube 36 is supplied in part by a diode 62, in tube 46, connected by way of a resistor 64 to the grid end of resistor'68. A predetermined negative bias is vsupplied to the control grid 44of tube 48 so that during desiredl operation rectification will not take place .in the circuit including diode 62. However, in the presence of unduly high modulating potentials in the input of tube 46, a negative potential is supplied by way of resistor 64 to the grids 32 and 34 to compress Y the range. Time constants of condenser 69 with resistors 64 and 68 are of such size'as to give a rapid rate of gain reduction and slow gain 're' covery.

In Fig. 3a, the modulating circuit is shown vsimply as a transformer V30' coupled to any source of modulation potentials on the one hand and on theV other hand to the suppressor grid elec-l I trode 24 of tube I0.

The frequency modulated output of thev arrangement of Fig. 3 is supplied to the grid 1 0 of tube 12 by way of a coupling condenser amplified therein and repeated in the tuned tank circuit 16.

In the simplified diagram of Fig; 3a, the out put electrode I6 of the electron-coupled generator and` modulator is connected to a tuned tank circuit I1 and supplied from saidcircuit to the desird frequency multipliers, amplifiers, etc. The oscillator I0.may operate at any desired frequency. In the arrangement used, the ascii--v lator of tube I0 operates at a frequency where f is the final rneanf-requencyA to be radiated.` The tank circuit 16 can then be tuned to frequencyl Y When this circuit is tuned to a-fraction of the final frequency, multiplication takes place in the later stages to bring the frequency up to the fre- `quency f. 1

In Fig. 3a, the oscillator circuit may operate at a frequency where N and n represent constants which result in a final desired output frequency f.

As stated above, oscillations of extremely stable mean frequency were obtained by an arrangement as illustrated in Figs. 3 and 3a without the use of automatic frequency ccJntrol,-compensatv ing reactances, etc. Careful construction, including the following precautions, were followed to -obtain the stability. A well regulated power source for thegoscillator tube i0 is assured by the use of a Voltage regulator tube 60, the operation and purpose of which needs no explanation. The winding LI is of Nilvar heavily coated with copper. Nilvar has a low thermal coeicient of expansion but is also of low Q. The Q was raised -by the copper coating. l In the apparatus .(Figs. 3 and 3a) tested, certain of the components thereof have the values listed below.

f is 39.64 megacycles C4 is 0.25 mmf.

2i is 370 mmf.

C2 is 40 mmf.

I9`is 25 mmf.

14 is 25 mmf.

52 is 25 mmf.

R2 is 100 ohms 25 is 400 ohms 23 is 20,000 ohms 29 is 20,000 ohms RFC is 2 m. h.

1l is 0.35 mmf.

` 13 is 2.5 m. h.

substantially the same as the modulator of Fig. l, 4

etc. The tube I0 in this modification, however, is ofthe direct heated filament type and in this ciru cuit I have includedchoking inductances |00 in the filament leads to prevent the radio frequencies flowing in the circuits of tube I0`from reaching the supply sources and modulation source. The f inductances |00 are preferably bi-filar wound with their terminals adiacent the filament 8 shunted by by-pass condenser |02. In this arrangement, as in certain of the priorv modications, the condenser C2 is the variable reactancev or swing reactance and thesize of 'this condenser determines the extent ofthe frequency deviation obtainable. Inthis modification, however, the resistance R2 has been omitted and the condenser |06 supplies the degenerative variable voltage in series with the voltage across condenser C2. Improved results are obtained in this particular arrangement by the use of a condenser which is relatively small in place of the resistor R2 heretofore used, the improved results being due toimproved phase relationship of grid and filament voltages. As in the prior modification, the condenser |06 permits the filament 8' to swing at radio-frequency voltage substantially in phase with the radio-frequency voltage on the grid 4 to obtain the degenerative effect and thereby control the effective size of the capacitive reactance C2. Thus, as in Fig. 1, with increased transcon-v ductance in the oscillator section of the tube I0, including the control grid 4, filament and screen grid electrode 20, there is a decreased effective capacitance C2 causing increased oscillation frequency.

In this modification, the screen grid potential is modulated through a blocking condenser ||0 connected with the output of an audio frequency amplifier. The audio amplifier comprises a transformer 30, the primary winding of which is coupled to amicrophone or television scanner output. The secondary winding of transformer is coupled to the control grid ||2 of an amplifier II4, the anode'of which feeds through a modulation potential modifying circuit I|6 to the grid IIB of tube |20. Tubes I|4 and ||6 may be in a single envelope and employ a common filament. That is, the filament of tube -I I4 may also represent the filament shown in tube |20.

The circuit IIS, as will be noted, is similar to the circuit between the secondary winding of transformer 42 and the grid 44 of tube 46 of Fig. 3. The components of this circuit have been labelled with reference numerals similar to the reference numerals used in Fig. 3 and a description thereof is believed unnecessary.

The diode element |22 of tube I|4 supplies a gain control potential to the grids ||2 and IIB in the presence of unduly high modulation peaks. The anode |24 of tube |20 supplies output to the resistors |26 and |28 and thence to coupling condenser ||0 and to the screen grid electrode 20 of tube I0. The anode |24 also supplies potential to the diode rectifier 22 by way of blocking condenser I30. Time delay circuits in the output of the diode rectier |22 include resistors |32 and |34 and condenser |36. Resistor |32 is of much in said output circuit is tuned to a multiple of lower value than |34, thus providing for a rapid gain reduction on excessive level peaks and a slow gain recovery after such high level peaks have ceased.

As in the modification shown in Fig. 2, the circuit comprising inductance LI and condenser C4 may be tuned to where J is the final frequency to be transmitted while the output circuit I1 of the tube I 0 may be tuned to a frequency anode, an anode and a cathode, a reactance and a radio frequency impedance connected between said grid and a point of lo'w radio-frequency po-y tential, a circuit connected between said electrode serving as an anode and said cathode, said last-j named circuit including said impedance, means for producing regeneration in said circuits whereby wave'energ'y is produced in said device andcircuits, means for modulating the potential' on an electrode in said device to thereby modulate the anode andcathode. y f

2. In a tube condenser, wave generator vice having a grid, an electrode serving`as an anode, an anode and a cathode, a condenserand a radio frequency impedance connected between said grid and a point of relatively low radio-fral quency potential, a circuit including said 'impedance connected betv'een said electrode serving as an anode and said cathode, means for producing regeneration in said circuits whereby wave energy is produced in said device, means for modulating the potential on an electrode of said device to thereby modulate the flow of current' through said impedance and vary the'voltage across said condenser, and an output circuit con-- nected with said anode and cathode.

3. An arrangement as recited in claim 2 where,-

the frequency of the wave generated.

4. In a wavelength modulation system, an electron discharge device having an anode, a cathode, a control grid, and a screen grid, wave generating circuits grid, cathode and screen grid electrode. said circuits including a reactance and a radio frequency impedance in series between said control grid and screen grid, means connecting said impedance between said screen grid and cathode, means connecting said impedance to a point of low radio frequency potential, means for producing regeneration in said circuits whereby wave energy is generated therein, means for modulating the potential on an electrode of said device to modulate the transconductance thereof and thereby modulate the effective reactance of said reactance between said control grid and screen grid, and an output circuit connected with said anode and cathode.

5. A system as recited in claim 4 wherein said output circuit is tuned to a multiple of the frequency of the wave energy generated.

6. In a wavelength modulation system, an-

electron discharge device having an anode, a cathode, a control grid, a screenA grid, and a suppressor grid, wave generating circuits interconnecting said control grid, cathode and 4screen grid electrode, said circuits including a react'- ance and an impedance in series between saidI control grid and screen grid, said impedance being also connected between said screen grid and cathode, means for producing regeneration in said circuits whereby wave energy is generated therein, means for modulating the potential on said suppressor grid to modulate the transconductance thereof and thereby modulate the ef fective reactance of said reactance between said control grid and screen grid, and an output circuit connected with said anode andl cathode.

7. Ina Wavelength modulation system, an elec tron discharge device having electrodes includ` ing a grid, a cathode, an anodeLa'nd anauxiliary and wavelength modulator, an electron discharge de-f interconnecting -said control asumo electrode connected in wave generating circuits including a reactance and a radio-frer quency impedance in series between the grid and a point of low radio-frequency potential, said generating circlliis also including said impedance between said a liary electrode and cathode, an output circuitcoupled to said anode and cathode and coupled to said wave generating circuits substantially by the electron stream only of said device, and means for modulating the transconductance of said device .in accordance with signais.

8. A system as recited in claim 'l wherein said output circuit is tuned to a. multiple of the irequency of the wave energy generated.

9. In a wavelength modulation system, a tube having a grid, a cathode, and an electrode serving as an anode, a reactance coupling said grid to said cathode, an unbypassed impedance coupiing said electrode serving as an anode to said vcathode whereby, if radio frequency is applied transconductance of said tube in accordance with signals to correspondingly vary the effective reactance between said grid and cathode and correspondingly vary the wave length of the'oscillations produced. v

10. A system as recited in claim 9 wherein said reactance is a capacity. y

11. A system as recitedl in claim 9 wherein said reactance is an inductance.

12. In a system for generating and wavelength modulating Wave energy, an electron discharge tube having a control grid, an electron serving as an anode and a cathode, a reactance and a radio-frequency impedance connected between said control grid and a point of low radio-frequency Ipotential, a circuit connected between said electrode serving as an anode and said cathode, said last-named circuit including said impedance, means for producing regeneration in said circuits whereby wave energy is produced in said tube and circuits, means for modulating the potential on an electrode in said tube to 'thereby modulate the ilow of radio-frequency current through said impedance and vary the voltage developed across said reactance and cony nections for deriving generated wave energy from said generating circuits.

13. An arrangement as recited in claim 12 wherein said reactance is a capacity.

14. An arrangement as recited in claim 12 wherein said reactance is an inductance.

J ARRE'IT LEWIS HATHAWAY. 

